Method for Controlling a Compressor Installation

ABSTRACT

A method for controlling a compressor system comprising a plurality of compressors, wherein the compressor system is intended to maintain a predefined excess pressure in a pressurized fluid system, wherein decisions are met at fixed or variable intervals as to switching operations for adapting the system to current conditions, wherein—in a pre-selecting step, switching alternatives are excluded from the plurality of combinatorially available switching alternatives,—in a main selecting step, remaining switching alternatives are weighed against one another while referring to one or more optimization criterion (criteria) and optimum switching alternatives are selected from among the given criteria, and—in a control step, the selected switching alternative is output for implementation in the compressor system.

The invention relates to a method for controlling a compressor systemcomprising a plurality of compressors, optionally of different designand/or performance. Concurrently, the invention relates to a controlmeans for such a compressor system as well as a data record for thecontrol of such a compressor system.

For the supply of sufficient pressurized fluid, compressor systems,optionally compressor systems provided for industrial applications,typically require a large number of individual compressors for supplyingthis pressurized fluid. In order to operate such a compressor systemefficiently, the cost effectiveness of the individual components as wellas the entire compressor system is not only increasingly taken intoaccount during the design and planning of the compressor system but alsoduring the operation thereof. These aspects as to the cost effectivenessof the compressor system are typically taken into account besidesenvironmental regulations and quality requirements which have to be metalso. The energy consumption of a compressor system can in thisconnection amount up to 80% of the total running costs, a reason why theenergy demand is the main cost factor for a compressor system operator.

To make use of the energy saving potentials of a compressor system, itwas found that apart from measures as to heat recovery or the reducingof leakages for instance, the use of appropriate controlling andregulating systems allows for a significant reduction of the operatingcosts of a compressor system. A control or regulation of the compressorsystem, for instance, enables various compressors to be suitably dividedup and consequently reduces the risk of failure respectively facilitatesmaintenance of the compressor system. In case of a compressor failing,for example, other compressors which are at no-load or stopped can beaddressed by the control or regulation and caused to provide pressurizedfluid so as to prevent the operational pressure of the compressor systemfrom dropping or the stopped status thereof.

In the simplest case, cascade or pressure band regulations are used forcontrolling compressor systems including a plurality of compressorswhich decide which compressor of the compressor system is in each caseswitched on or off at predefined operating conditions. In the cascaderegulation, each compressor is assigned a determined pressure rangeaccording to which the switching on or off of a respective compressor isdetermined by the control. Thanks to this definition of individualpressure ranges, also called pressure bands, which are assigned to thecompressors, the demanded amount of pressurized fluid can be coveredeven at high withdrawal rates of pressurized fluid by switching on alarger number of compressors respectively by switching on compressorshaving an increased delivery amount as compared to the others. Adisadvantage in such regulations, however, is that the currentconsumption of pressurized fluid or the change of the current withdrawalof pressurized fluid typically is not taken into account.

Improved pressure band controls make use of the possibility to controlany desired number of compressors via a single pressure band. Such amethod of controlling can achieve the reduction of the maximum pressureof pressurized fluid prevailing in the compressor system, on the onehand, and simultaneously also decrease some energetic losses in thecompressor system, on the other.

Yet, it has shown that pressure band regulation at a typical graduationof individual compressors relative to each other at a fluctuatingwithdrawal of pressurized fluid from the compressor system are notsuited to control a compressor system such that the demand forpressurized fluid can be covered sufficiently, on the one hand, and, onthe other, in an energetically efficient manner. For example,operational states or constellations can arise in the compressor systemwhich either lead to an insufficient supply of pressurized fluid or toan energetically extremely inefficient supply of pressurized fluid.According to these disadvantages known from prior art, hence the task isto propose an improved method for controlling a compressor system whichenables a sufficient supply of pressurized fluid even at fluctuatingwithdrawal of pressurized fluid from the compressor system, whereinconcurrently the switching operations caused by the control should be aseconomic as possible.

According to the invention, this task is solved by a method forcontrolling a compressor system which comprises a plurality ofcompressors, optionally of different design and/or performance,according to claims 1 and 3. Moreover, the task is solved by acontrolling means for such a compressor system according to claims 25and 26, as well as by a data record for controlling such a compressorsystem according to claims 27 and 28.

The task on which the invention is based is optionally solved by amethod for controlling a compressor system which comprises a pluralityof compressors, optionally of different design and/or performance,wherein the compressor system is intended to maintain a predefinedoverpressure in a pressurized fluid system despite a possibly evenfluctuating withdrawal of pressurized fluid from the pressurized fluidsystem, wherein decisions are met at fixed or variable intervals as toswitching operations for adapting the system to current conditions,wherein in a pre-selecting step, preferably in consideration of thecurrent conditions, switching alternatives are excluded from themultitude of combinatorially available switching alternatives, whereinin a main selecting step remaining switching alternatives are weighed upagainst one another while referring to one optimization criterion ormore optimization criteria, and optimum switching alternatives areselected among the given criteria, and wherein in a control step, theselected switching alternative is output for implementation in thecompressor system.

The task is moreover solved by a method for controlling a compressorsystem which comprises a plurality of compressors, optionally ofdifferent design and/or performance, wherein the compressor system isintended to maintain a predefined overpressure in a pressurized fluidsystem despite a possibly even fluctuating withdrawal of pressurizedfluid from the pressurized fluid system, wherein the control of thesystem takes measures for increasing the generation of compressedpressurized fluid upon reaching a possibly variable switch-on pressure,and for reducing the generation of compressed pressurized fluid uponreaching a switch-off pressure, wherein the switch-off pressure isvariable and can be changed as a function of the current configurationof the compressor system and/or in consideration of a defined switchingoperation (a defined change of the compressor system configuration).

Here and in the following, the maintenance of a predefined overpressureshall be performed such that an adaptation pressure that can be reachedby the real pressure process is not or only insignificantly and/orshortly undercut, and optionally an upper pressure limit is not or onlyinsignificantly and/or shortly exceeded.

The inventive task is in addition solved by a controlling means for acompressor system which comprises a plurality of compressors, optionallyof different design and/or performance, wherein the compressor system isintended to maintain a predefined overpressure in a pressurized fluidsystem despite a possibly even fluctuating withdrawal of pressurizedfluid from the pressurized fluid system, wherein decisions are met atfixed or variable intervals as to switching operations for adapting thesystem to current conditions, and wherein the controlling meanscomprises: an excluding means which excludes, preferably inconsideration of the current conditions, switching alternatives from amultitude of combinatorily available switching alternatives, a selectingmeans which weighs up remaining switching alternatives against oneanother while referring to on optimization criterion or moreoptimization criteria, and selects an optimum switching alternativeamong the given criteria, as well as an output means which is configuredto output the selected switching alternative for implementation in thecompressor system.

The task on which the invention is based is further solved by acontrolling means for a compressor system which comprises a plurality ofcompressors, optionally of different design and/or performance, whereinthe compressor system is intended to maintain a predefined overpressurein a pressurized fluid system despite a possibly even fluctuatingwithdrawal of pressurized fluid from the pressurized fluid system, andwherein the controlling means comprises: a switch-off pressuredetermining means which, upon overproduction of pressurized fluid,determines a switch-off pressure as a function of the currentconfiguration of the compressor system and/or in consideration of adefined switching operation (a defined change of the compressor systemconfiguration).

The inventive task is further solved by a data record which ispreferably configured for transmission in a data network or stored on adata carrier, for controlling a compressor system, wherein thecompressor system comprises a plurality of compressors, optionally ofdifferent design and/or performance, and wherein the compressor systemis intended to maintain a predefined overpressure in a pressurized fluidsystem despite a possibly even fluctuating withdrawal of pressurizedfluid from the pressurized fluid system, wherein decisions are met atfixed or variable intervals as to switching operations for adapting thesystem to current conditions, wherein in a pre-selecting step,preferably in consideration of the current conditions, switchingalternatives are excluded from the multitude of combinatoriallyavailable switching alternatives, wherein in a main selecting stepremaining switching alternatives are weighed up against one anotherwhile referring to one optimization criterion or more optimizationcriteria, and optimum switching alternatives are selected among thegiven criteria, and wherein in a control step, the selected switchingalternative is output for implementation in the compressor system.

The inventive task is moreover solved by a data record which ispreferably configured for transmission in a data network or stored on adata carrier, for controlling a compressor system, wherein thecompressor system comprises a plurality of compressors, optionally ofdifferent design and/or performance, and wherein the compressor systemis intended to maintain a predefined overpressure in a pressurized fluidsystem despite a possibly even fluctuating withdrawal of pressurizedfluid from the pressurized fluid system, wherein the control of thesystem takes measures for increasing the generation of compressedpressurized fluid upon reaching a possibly variable switch-on pressure,and for reducing the generation of compressed pressurized fluid uponreaching a switch-off pressure, wherein the switch-off pressure isvariable and can be changed as a function of the current configurationof the compressor system and/or in consideration of a defined switchingoperation (a defined change of the compressor system configuration).

Here and in the following, the term control shall also be understood inthe meaning of a regulation. Since the method for controlling acompressor system as well as the individual embodiments of the methodcan exhibit both control-specific and regulation-specific features, astringent discrimination of bother terms was presently relinquished infavor of an understandable legibility.

A core idea of the present invention is to take in each case intoaccount a multitude of possible switching alternatives prior toimplementing a switching operation for adapting the pressurized fluidsystem to current conditions, which switching alternatives are weighedup against one another while referring to one optimization criterion ormore optimization criteria so as to be able to select a best possibleswitching alternative for implementation. In doing so, numerous possibleswitching alternatives can be excluded due to the use of a pre-selectingstep prior to realizing the main selecting step, whereby subsequentlyonly just a smaller number of possible switching alternatives has to becompared against each other. This separation of different selectingsteps permits a relatively rapid selection of a best possible switchingalternative which is subsequently output in a control step via aswitching command for implementation in a compressor system.

As a consequence, switching operations can be performed in shorter andconsecutive time intervals, whereby an improved adaptation of thepressurized fluid system to current conditions of the compressor systemcan be achieved. As a further consequence, the cost effectiveness of thecompressor operation is increased. If an important withdrawal ofpressurized fluid from the pressurized fluid system takes place, forexample, it is possible for the compressor system control by performingthe pre-selecting step to avoid unnecessarily complicated weightings bycomparing a relatively large number of possible switching alternativeswhile referring to one or more optimization criteria, and to restrictthe weighting to a smaller number of possible and suitable switchingalternatives. Consequently, it is possible for the present control torespond in a very short time with a nonetheless suitable and bestpossible switching alternative to an important withdrawal of pressurizedfluid from the pressurized fluid system.

A further core idea of the present invention is that the control of thecompressor system takes measures upon reaching a switch-off pressure forreducing the generation of compressed pressurized fluid, wherein theswitch-off pressure is variable. The present control accordingly differsfrom a typical pressure band control known from the prior art in whichswitching operations as a rule are triggered upon reaching fixedpredefined pressure values. The variable design of the switch-offpressure allows for a suitable adaptation of an actuating operation tothe current configuration of the compressor plan, respectively can alsotake into account defined actuating operations according to a definedchange of the compressor system configuration.

Essential reasons for the inefficiency of use of a compressor systemusing a typical pressure band control can be that, on the one hand, atoo large given pressure band temporarily leads to unnecessarily highpressures in the pressurized fluid system, whereby the compressors underload have to do unnecessary work. On the other hand, a too small givenpressure band can lead to unnecessarily frequent switching operationswhich to a large extent can result in unnecessary work associated withthese switching operations.

Hereinbefore and hereinafter, a switch-on pressure shall be understoodas a virtual pressure value, upon reaching of which the control of thecompressor system causes switching operations to be implemented whichcounteract dropping of the overpressure prevailing in the pressurizedfluid system. The switch-on pressure hence is below the switch-offpressure which is likewise defined as a virtual pressure value, uponreaching of which switching operations are likewise caused in thecompressor system at an increasing real pressure profile which result incompressors being switched off. Switching on as well as switching offcompressors here can comprise not only switching on or off the entirecompressor unit into load running or no-load running respectivelystopped state but also a gradual changing of the output to higher orlower values.

In accordance with the switching alternative implemented in thecompressor system, a changing profile of the prevailing pressure(overpressure) in the compressor system comes about. This pressureprofile which constitutes a really measurable parameter exhibits localminimum and local maximum values during its time course which resultfrom the withdrawing of pressurized fluid from the pressurized fluidsystem or the supplying of pressurized fluid by the respectivecompressors. Typical switching operations to be executed upon exceedingthe switch-off pressure are the switching of a compressor or compressorgroup from load to no-load running or stopped status or else thereduction of the running power of load running compressors or compressorgroups. Typical switching operations to be implemented in the compressorsystem upon falling below the switch-on pressure which is lower ascompared to the switch-off pressure, are the switching of a compressorat stopped status or no-load running to load running or else theincreasing of the running power of load running compressors orcompressor groups in order to achieve the increased conveying ofpressurized fluid.

Due to the technical structural characteristics of compressors,switching operations upon exceeding the switch-off pressure essentiallyare implemented immediately. Switching operations which are realized atan overpressure decreasing in the compressor system and falling belowthe switch-on pressure, however, typically are only implemented at acertain time delay (dead time), since a start-up of a compressor fromstopped status or no-load running to the desired operating speedrequires a technically necessary preliminary run. Accordingly, suchpre-running times are shorter upon switching off a compressor ascompared to a switching on of a compressor, the two switchingoperations, however, lead to an implementation of the induced switchingoperations which is typically staggered in time.

Accordingly, the virtual switch-off pressure in practice is identical toa local maximum value of the real pressure profile to be reached. It istrue that exceptions are possible in case of very rapid reductions ofthe pressurized fluid demand and/or erroneous selection of too smallcompressors to be switched off load, but arise seldom in practice. Incontrast to this, the virtual switch-on pressure as a rule issignificantly above the virtual adaptation pressure to be reached towhich the minimum value of the real pressure course should correspond,since although switching-on operations are caused to be implemented uponfalling below the switch-on pressure, same can only start the fullpressurized fluid supply staggered in time due to the compressors'immanent delay times

A task of the present method for controlling a compressor system henceis to determine the switch-on pressure such that the minimum values ofthe real pressure profile reach the adaptation pressure as precisely aspossible but do not fall below same. In other words, the adaptationpressure is a virtual pressure value which the minimum values of thereal pressure profile should reach as precisely as possible. Theadaptation value hence is a default value for a real pressure value notto be undercut, if possible, which in one possible embodiment can bevariably assessed as a function of the current operating state of thecompressor system.

To comply with the pressure resistance limits of the components withinthe pressurized fluid system it is typically also necessary for the realpressure profile to not exceed an upper pressure limit, if possible.Consequently, upon reaching the upper pressure limit at the latestappropriate switching operations are triggered in the compressor system,for example, the switching off of compressors under load, so that thereal pressure profile does not exceed the upper pressure limit, ifpossible. In practice, the upper pressure limit usually can be set to beso high as to be above the switch-on pressures which are determined on acase-by-case basis due to criteria for minimizing the energyconsumption, so that the switch-off pressures and the maximum values ofthe real pressure profile which largely correspond thereto, come aboutwithout influencing the upper pressure limit and hence within thepressure tolerance between adaptation pressure and upper pressure limit,preponderantly or exclusively from energetic aspects.

If the virtual switch-on pressure for individual switching operations isassessed on a case-by-case basis so that the real pressure profilereaches the adaptation pressure at a decreasing pressure profile asprecisely as possible, this has positive effects on the energyconsumption of the entire compressor system since an unnecessaryincrease of the pressure level due to the compressors being switched ontoo early is prevented and an unnecessary work performance does notoccur.

It is to be noted at this point that the determining of the variablevirtual switch-on pressure by the control of the compressor system takesplace such that the minimum values of the real pressure profile reachthe given adaptation pressure as precisely as possible but do not oronly insignificantly and/or shortly fall beyond same. For this purpose,the switch-on pressure is determined such that a switch-on response timeof a compressor or a compressor group to be switched to load follows aprognosticated pressure profile. The determining of the switch-on orswitch-off pressure by the control of the compressor system can alsoensue by a control of the compressor system on a time basis instead of apressure basis, with the determining of the switch-on or switch-offpressure being replaced by appropriately determining a switch-on orswitch-off time. A control based on a time basis hence is equivalent tothe present control based on a pressure basis. The determining of aswitch-on time, alike the determining of a switch-on pressure (the sameapplies to the switch-off time or switch-off pressure), is in each caseexecuted case-by-case for future switching operations.

Furthermore, it is to be noted that the method provided for supplying apredefined overpressure in a pressurized fluid system can also be usedanalogously in a vacuum system in which a negative pressure not to beexceeded must be maintained which is made available to users. Switchingon a pump comprised by a corresponding system consequently would entaila pressure drop of the pressurized fluid in the system, and switchingoff a pump or a pump group accordingly would entail an increase of thepressure in the pressurized fluid system in case of vacuum beingwithdrawn or vacuum deteriorating e.g. due to leakages. According to theskilled person's understanding, a transfer of the present method forcontrolling a system to maintain a predefined overpressure to a methodfor controlling a compressor or pump system in which a predefinednegative pressure should not be exceeded, can be realized analogously.

One preferred embodiment of the method for controlling a compressorsystem provides for the control of the system, upon reaching a possiblyvariable switch-on pressure, to take measures for increasing thegeneration of compressed pressurized fluid and, upon reaching a possiblyvariable switch-off pressure, measures for reducing the generation ofcompressed pressurized fluid.

A further preferred embodiment of the method for controlling acompressor system provides for the switch-off pressure to be assessed,optionally calculated, in an energy optimization on a case-by-casebasis. Accordingly, the compressor systems are primarily controlled withthe objective of optimizing, i.e. minimizing the energy demand, whereina predefined overpressure is concurrently maintained, i.e. preferablynot or only insignificantly and/or shortly undercut. Optimizingrespectively minimizing should be understood hereinafter merely as anoptimizing respectively minimizing within the scope of the possibleswitching alternatives. Due to this energy demand minimizing, thepresent method differs significantly from conventional pressure bandcontrol methods which control the pressure in the pressurized fluidsystem as the most important control parameter rather than the energydemand of the compressor system. A targeted energy saving can berealized due to the exploitation of the technical degrees of freedomwhich are made available by selecting a suitable switching alternativefrom a multitude of different switching alternatives. Optionally thevariable switch-on pressure can be determined in this case such that,upon the real pressure profile falling below the switch-on pressure, thevirtual adaptation pressure corresponds in the reversal point of thereal pressure profile as precisely as possible to the minimum values ofthe real pressure. Such an optimization, on the one hand, allows thewanted pressure level to be maintained, and, on the other, the number ofnecessary switching operation to be kept low, which results in a verycost-effective operation.

In a further embodiment of the method according to the invention, theoptimum switch-off pressure is determined by computationally minimizingthe quotient from the total work loss in a predefined periodic timeinterval concerning one switching alternative, and the time intervalitself. In this case, the total work loss comprises the sum of work lossof all load running compressors in said time interval, the no-loadrunning work loss of all of the compressors to be switched on in thesaid time interval, and the switching work loss of all of thecompressors to be switched on and off in said time interval. Theperiodic time interval is in this case based on the observation ofso-called switching cycles. Such (virtual) switching cycles aretime-pressure-profiles which similarly (periodically) repeat within thetime interval (switching cycle duration) rising form a minimum to amaximum and falling again to a minimum pressure value which would ariseat a temporarily essentially constant withdrawal of pressurized fluid,i.e. at least for the switching cycle duration. For a simplifyingcalculation it may be assumed that the withdrawal of pressurized fluidfrom the pressurized fluid system takes place such that the realpressure profile between the minimum and maximum pressure value can beassumed in each case to be linear or approximated by straight lines. Thecompressor(s) to be switched to load upon reaching the switch-onpressure, respectively off load upon reaching the switch-off pressureis/are presupposed to be known and can also be selected beforehand bymeans of appropriate heuristics. It is moreover also typically assumedthat the operating states of the remaining compressors are onlyinfluenced by the pressure profile of the real pressure and otherwiseremain unchanged.

A switching cycle concerns a period length of the real, likewiseperiodic pressure profile. The simplifying assumptions beingpresupposed, the mean energy demand of all of the compressors comprisedby the compressor system during one switching cycle, i.e. during thepreviously described periodic time interval, can be minimized in oneclosed mathematical expression. In doing so, however, the observation ofthe total mean energy demand of all compressors is not required, rathercan an appropriately defined total power loss P_(V) be assumed as asimplification which is treated substitutionally. In the simplest case,this power loss can be calculated from the previously described totalwork loss as well as the length of the periodic time interval of oneswitching cycle as a quotient. The thus defined total power loss is atemporally averaged power loss within one switching cycle. As will beexplained in more detail below, a simple mathematical manipulationallows for an optimized switching cycle pressure difference to becalculated which can be derived from parameters which are simple todetermine. The switching cycle difference is defined by the differencebetween the switch-off pressure and adaptation pressure.

Tests have shown that control or regulating methods which refer to anoptimization of the switching cycle pressure difference as an optimizingcriterion, have achieved considerable successes in reducing the energyconsumption of the compressor system.

A further development of the method of the invention can moreoverprovide for the following parameters to enter into the calculating ofthe switch-off pressure: the energy demand of the load runningcompressors, optionally with a conveying against a continuouslyincreasing pressure and/or no-load running losses of the compressors tobe switched to no-load running or stopped status and/or no-load runninglosses of the no-load running compressors and/or switching loss energyof the compressors to be switched per switching alternative. In doingso, the parameters concerned can be established according to knownheuristics or else be determined in appropriate tests respectively bymeans of appropriate calculating methods. They can optionally alsoinclude the temporal behavior of the individual compressors in the formof time charts for all load, no-load or switching states in aquantitative form, wherein the time delay between one switching time andthe complete implementation of a switching operation can also beexplicitly taken into account. The delay times therefore can also enterinto the determining of an appropriate switch-on or switch-off pressureas a calculating parameter.

In accordance with the embodiment it is also possible for the switch-onpressure to be calculated in the method for controlling a compressorsystem such that the real pressure profile reaches as precisely aspossible a calculated adaptation pressure, which is below the switch-onpressure, preferably at a deviation of less than 5%, further preferredless than 2%, and further preferred does not or only insignificantlyand/or shortly fall below same. Correspondingly, the maintenance of apredefined overpressure can be ensured in the compressor system, whereina cost-effective and efficient control of the compressor system takesplace concurrently.

A further embodiment of the method according to the invention providesfor the switching alternatives for the reducing of the generation ofpressurized fluid to be evaluated according to optimizing criteria otherthan switching alternatives for the increasing of the generation ofpressurized fluid. A further differentiated adaptation of the method ofthe invention can accordingly take place, whereby it can be reached, forexample, that the real pressure profile in its reversal points, i.e. itsminimum and maximum pressure values, reaches as precisely as possiblethe predefined adaptation pressure and the switch-off pressure which iscalculated or determined on a case-by-case basis according to criteriaof energy consumption optimization during one switching cycle duration.

In a further developed embodiment of the method according to theinvention for controlling a compressor system, decisions are met as tothe weighting and selecting of switching alternatives for the reducingof the generation of pressurized fluid among optimizing criteria whichprimarily or exclusively take into account the respective total energyexpenditure of the different switching alternatives under consideration.

In a further developed embodiment, the taking into account of the totalenergy expenditure of different switching alternatives includes atleast: the energy demand of the load running compressors and/or no-loadrunning losses of the compressors to be switched in idle running or atstopped status and/or no-load running losses of the idle runningcompressors and/or switching loss energy of the compressors to beswitched per switching alternative. Since the total energy expenditureis calculated in an optimized manner on a case-by-case basis over alltime periods of the utilization of the compressor system and entersdirectly into the selection of an appropriate switching alternative, aparticularly energy-efficient control of the compressor system isachieved.

In accordance with the embodiment, the evaluating and selecting of theswitching alternative can take place in real-time. Here and hereinafterreal-time will be understood as a time dimension which is considerablyshorter than the time sequence of two switching alternatives to beimplemented. Accordingly, the evaluating and selecting of the switchingalternative takes place at a sufficient speed so as to be able to alsotake into account unexpected important changes of the pressurized fluidprovided in the pressurized fluid system. In other words, the delaycaused by the evaluating and selecting of the switching alternative isnot required to be explicitly taken into account in the control method.

In another embodiment of the method according to the invention, thedetermining of the switch-off and/or switch-on pressure is performed inreal-time. Accordingly, an immediate adaptation of the control to thechanging operating states in the pressurized fluid system can beperformed at a sufficient speed without fundamentally new operatingstates arising during the time required for the determining of theswitch-off and/or switch-on pressure which would necessitate theselecting of another switching alternative.

In a further preferred embodiment of the method according to theinvention, the control of the system is performed in consideration ofexperiential parameters from past switching operations (adaptivecontrol). The control can optionally determine the switch-on pressuresuch that the production start of a compressor switched to load isperformed sufficiently early so as to make the pressure reversal of thereal pressure profile happen as close as possible to the adaptationpressure. In doing so, the control method can adaptively learn aswitch-on response time for each compressor which has to be understoodas a time span between the transmission of a switch-on command forimplementing a switching alternative and the actual start of the effecton the real pressure profile. The switch-on pressure can be selectedsuch that the switch-on response time equals the time span in which thereal pressure profile is expected to drop from the switch-on pressure tothe adaptation pressure. This time span can be estimated by a prognosisof the further pressure profile based on suitable assumptions, forexample, based on the assumption of a linearly dropping pressureprofile.

An adaptive learning of the switch-on response time for each compressorcan be performed inter alia by evaluating the real pressure profilesover a number of selected periodic time intervals of the real pressureprofile of one compressor or a group of compressors. The adaptivelylearnt switch-on response times can be further updated, evencontinuously, by appropriately forming new values, e.g. moving averagevalues.

In doing so, the adaptive learning behavior of the control supportsdecisively the target to optimize the energy demand of the compressorsystem. The adaptive behavior is in this case typically based onunderlying learning algorithms and adaptive parameters, which parametersare readjusted by the control in the course of the control method, andare available from the control in an updated manner for any furtherevaluation and selection of a switching alternative. Consequently, theadaptive learning behavior permits the control to automatically adapt toany regulation-technologically relevant characteristics and conditionsof the compressor system during running operation. Sinceapplication-technologically relevant parameters can be also collectedand evaluated (level of energy demand), the control flexibly adapts tothe behavior of the compressor system in the operating state in terms ofan energetic optimization.

The underlying learning algorithms can calculate the designated adaptiveparameters either by evaluating a measurement value which was tracedover a longer period of time or by evaluating a suitable number ofsingle events. Both approaches are suited to readjust the adaptiveparameters during the running operation of the compressor system whilekeeping short-term influences or singular influences out of thecalculating of the adaptive parameters.

The adaptive behavior of the control permits to get along with onlyrelatively few control parameters, wherein the control behavior of thecontrol is not required to be manually optimized or re-optimized andeven with expansions or constructional alterations of the compressorsystem any further adaptations are not required to be done. Theessential parameter is in this case typically the adaptation pressure,whereas the switch-off pressure or the switching cycle pressuredifference will result from the switch-off pressure and adaptationpressure based on criteria with respect to minimizing the energy demand.Accordingly, the operating-technological and maintenance-technologicalexpenditure for the startup and maintenance of the control is minimal.

In a further developed embodiment of the method according to theinvention, the experiential parameters comprise the level of energydemand (energy demand per fluid quantity) of individual compressors orcertain combinations of compressors and/or switch-on response time ofthe compressors and/or consumption behavior of the pressurized fluidconsumers and/or the size of the pressure accumulator and/or thepressure compensation of the compressors or certain combinations ofcompressors.

The level of energy demand describes the energy utilization of singlecompressors or combinations of compressors in the running operation asan adaptive parameter and is represented as a ratio of energy demand andthe fluid amount conveyed through the involved compressors. In thiscase, the energy demand as well as the conveyed fluid amount arecalculated over an appropriately selected period of time by numericallyintegrating the power consumption or conveyed amount which have beenmade accessible on a computational and/or metrological way. Since thecomputing of the level of energy demand describes all of the actuallyperformed works (load work, no-load running, work loss, switching workloss) as well as the actually conveyed fluid amount at sufficientaccuracy, the level of energy demand, in contrast, for example, tovalues calculated from purely theoretical nominal values of thecompressors, can reflect the actual energetic utilization in the runningoperation in a relatively precise manner.

In doing so, it can be taken into account in the control that forcompressors or groups of compressors which due to past energeticallyunfavorable load cycles exhibit a correspondingly lower energeticutilization and probably are untruly long classified in the selection ofthe aggregates to be switched on load as being low-level (positivefeedback), the level of energy demand is successively adapted to currentenergetic utilization characteristics of the compressor system by acompensation mechanism.

In this case, it should also be noted that, when switching to load,typically compressors which are at no-load and have a comparativelylarge remaining no-load power, are preferred to compressors which have acomparatively smaller remaining no-load work or the motor of which isalready switched off in order to save energy by avoiding no-load workloss and startup work. Moreover, when switching from load, typicallythose among compressors of equal or similar size are preferred whichhave an expected small no-load work loss in order to thus save energy byavoiding no-load work.

The pressure-technological effect of the individual compressors on thecontrol is described in the form of a pressure compensation degree ofthe compressors as an adaptive parameter and can be determined via thepressure compensating effect of switching operations by averaging overan appropriate number of single events. In this case, the pressurecompensating effect of the switching operations can be taken from thechronological pressure variation.

Since in the selecting of compressors to be switched, preferably onlycompressors or groups are taken into account the pressure compensatingeffect (sum of the pressure compensation degrees) of which is adapted tothe current operating state (current pressure profile) of the compressorsystem, the switching of the selected compressors typically induces thedesired pressure profile to be established in time so that in practiceadditional, energetically disadvantageous switching operations are notrequired.

In operating states which are characterized by a rapid change in thepressurized fluid reduction, compressors can be selected under definedconditions, the pressure compensating effect of which cannot completelycompensate the real pressure profile in terms of a reversal of thepressure direction, which constitutes an undercompensation of the realpressure profile at the switching time. In such cases, the control cantherefore advance the switching time by a time span which is adapted tothe extent of the undercompensation. In accordance with the embodiment,a time buffer is provided so as to switch further compressors in time,if needed, whereby it can be achieved that no further compressors atbest need to be switched or that, after a switch-on operation, thepressure can at least be stabilized on an energetically favorable levelfor a relatively longer period of time.

Moreover, it can happen in very rare cases that the adaptation pressureis inadmissibly undercut under conditions of strong fluctuations ofpressurized fluid being withdrawn from the compressor system. In suchsituations, the control can counteract the deviation of the realpressure profile from the adaptation pressure as required immediatelyand situation-adapted by immediately switching one or more additionalcompressors to load. Still during the running switch-on operation, i.e.before the pressure compensating effect of the switched-on compressorshas started, it can be checked on the basis of the real pressure profilewhether the future pressure compensating effect is expected to besufficient to establish a desired real pressure profile. If the futurepressure compensating effect is determined to be sufficient, no furthercompressor will be switched to load. Otherwise, one or more compressorsare immediately switched to load.

In an alternative embodiment of the method according to the invention,the experiential parameters can comprise: the pressure compensationdegree of a compressor depending on the storage volume and theinstallation scheme of the pressurized fluid system and/or the level ofenergy demand of a compressor depending on its previous mode ofoperation, environmental temperature, maintenance, wear andcontaminating states and/or the switch-on response time and pressurecompensation degree of a compressor depending on typical patterns of thechange of withdrawal of pressurized fluid. Accordingly, the control canalso adaptively learn not purely compressor-specific characteristics butalso in part characteristics which result from the interaction ofcompressors and the operating state or the respective environment ofuse.

In a further preferred embodiment of the method according to theinvention, same is characterized in that a switching on of compressorsor a combination of compressors is performed early enough so that inconsideration of the startup behavior of the compressor or thecompressor combination, optionally in consideration of the preferablyadaptively learnt switch-on response times, the real pressure profilereaches the adaptation pressure as precisely as possible, preferably ata deviation of less than 5%, further preferred less than 2%, and furtherpreferred does not or only insignificantly and/or shortly fall belowsame. Accordingly, the real pressure profile reaches in its minimumpressure values within relatively narrow limits the virtually determinedadaptation pressure as precisely as possible.

A further embodiment of the method according to the invention canprovide for preferably selecting such compressors or combinations ofcompressors in the determining of switching alternatives of compressorsto be switched to load, which have favorable values of experientialparameters for the level of energy demand. Accordingly, an energy-savingoperation as possible of the compressor system is guaranteed.

In another embodiment of the method for controlling a compressor system,preferably such compressors are selected in the determining of switchingalternatives of compressors to be switched to load which are at no-loador still have a long remaining no-load running time or remaining no-loadwork, and/or such compressors are preferably selected as compressors tobe switched to no-load or stopped status which have a low no-loadrunning time or no-load work loss. Accordingly, the total work loss as asum of all work losses is also reduced since a reducing of the no-loadpower is taken into account in the determining of the switchingalternative to be selected in term of energetic optimization.

Another preferred embodiment of the method according to the inventionprovides for the determining of switching alternatives and/or thedetermining of a switch-off pressure and/or the determining of aswitch-on pressure to be performed assuming a constant pressurized fluidreduction. In this case, the assumption of a constant pressurized fluidreduction is only reasonably performed for the determining of therespective next switch-on or switch-off pressure. For the subsequentdetermining of a future and following switch-on or switch-off pressure,a new, in turn constant value for the pressurized fluid reduction istaken as a basis, as need be. This assumption of a constant pressurizedfluid reduction allows the real pressure profile during a switchingcycle including the next switch-on operation to be calculated usingmathematically simple to handle expressions in terms of energy.Consequently, en energetic optimum or a maximum efficiency related tothe operation of the compressor system can also be calculated for theperiod of a switching cycle.

In a further embodiment of the method according to the invention, thedetermining of a current value of the withdrawal of pressurized fluid iseither determined by a measuring device and/or calculated from the pastreal pressure profile, the operating state of the compressors and/or thepossible adaptively learnt accumulator size of the compressor system.

In a further embodiment of the method according to the invention, sameis characterized in that, under predefined conditions, the switch-on orswitch-off commands to be triggered upon reaching the switch-on pressureor switch-off pressure can be suppressed and/or additional switch-oncommands or switch-off commands triggered independent of the reachingthe switch-on pressure or switch-off pressure. Additional switch-offcommands can thus be triggered, for instance, upon approaching the upperpressure limit so as to prevent the real pressure profile from exceedingthe upper pressure limit. Moreover, a determined switch-on command canbe suppressed upon a distinct and lasting positive curvature of thedecreasing pressure profile due to a reducing withdrawal of pressurizedfluid from the pressurized fluid system so as to be able to betterestimate the further real pressure profile. Accordingly, switch-offcommands can also be suppressed upon a distinct and lasting negativecurvature of an increasing real pressure profile which results fromincreasing pressurized fluid being withdrawn. Here as well, theswitching alternative selected by the control for performing an improvedenergetic calculation is initially suppressed so as to be able to betterestimate the further pressure profile and consequently to perform animproved future switching operation with respect to the energyconsumption.

When in another, likewise preferred embodiment of the method accordingto the invention, a plurality of switching alternatives are determinedas being energetically equivalent further criteria, such as the numberof operating hours of a compressor in question, same are additionallytaken into account. Accordingly, it can be guaranteed that the number ofoperating hours of different compressors comprised by the compressorsystem is mostly uniform, whereby maintenance-contingent oruse-contingent outages of single compressors can be reduced to apredetermined extent.

In accordance with a further embodiment of the method according to theinvention, the determined switching off is only released by the controlwhen it is ensured that a possibly necessary switch-on operation can beperformed in time in consideration of the startup behavior of a possibleswitching combination. Such a considering of the startup behavior of acompressor or a combination of compressors of the compressor systemallows for a predefined overpressure to be always maintained in thepressurized fluid system. An unforeseen and in the normal caseenergetically unfavorable switching on of further compressors orcompressor groups due to the releasing of a determined switchingoperation which cannot be implemented in time, can thus be avoided.

Further embodiments of the invention result from the subclaims.

The invention will be described in more detail below on the basis ofexemplary embodiments which will be explained by means of the figures.

Shown are in:

FIG. 1 a schematic representation of a compressor system comprising aplurality of compressors;

FIG. 2 a schematic representation of one embodiment of the control meansaccording to the invention for controlling the compressor system shownin FIG. 1;

FIG. 3 a schematic representation of one embodiment of the controlaccording to the invention in a flowchart representation;

FIG. 4 a representation of a real pressure profile in a compressorsystem indicating specific control parameters according to oneembodiment of the control method according to the invention.

FIG. 1 shows a schematic representation of a compressor system 1 whichcomprises six compressors 2 in total each connected to a communicationsbus 5. Via appropriate pressure lines, each of the compressors 1 isconnected to processing elements 21 which can be realized as dryers orfilters, for example. The six compressors 2 fluidically supply a centralpressurized fluid reservoir 3 which additionally has a measuring means20 which is communicatively coupled to the communications bus 5. Themeasuring means 20 allows in this case, for example, the pressure statewithin the pressurized fluid reservoir 3 to be measured continuously andis capable of forwarding measured parameters to the control of thecompressor system 1 via the communications bus 5 which are available inthe control method 41 (presently not shown) in terms of controlengineering.

The pressurized fluid supplied by the compressors 2 in the pressurizedfluid reservoir 3 is forwarded to a user for withdrawal of pressurizedfluid via an appropriate pressure line which can alternatively comprisefurther functional elements 22 (in the present case e.g. a controlvalve). The control or regulating of the overpressure maintained withinthe pressurized fluid reservoir 3 is performed by means of a centralcontrol means 4 which is presently not shown but is communicativelycoupled to the communications bus. In this case, the communicationbetween the compressors 2 and the communications bus 5 can take placevia conventional wired signal lines or else via wireless communicationpaths.

In accordance with the embodiment, the selected communication protocolcan ensure the control method explained below in more detail to beexecuted in real-time. The pressure prevailing within the pressurizedfluid reservoir 3 is detected by the measuring means 20 preferablylikewise in real-time. Practically, a sampling in time intervals of lessthan one second, preferably less than a tenth of a second is suitablefor this purpose. In typical pressurized fluid applications, themeasuring means 20 will measure an overpressure within the pressurereservoir 3. In also possible vacuum applications, the measuring means20 will measure, as described above, a corresponding negative pressurewhich can likewise be provided in the pressurized fluid reservoir 3. Aswill be clear to the skilled person, the compressors 2 are for thispurpose replaced by appropriate vacuum pumps. The pressure valuedetected by the measuring means 20 can be more or less smoothed,evaluated on an absolute or time-differential or combinatory basisdepending on the purpose of use, in order to being introduced in thecontrol or regulating method. The thus conditioned pressure value can beused inter alia for calculating an energetically optimum switch-offpressure 103 (presently not shown), for calculating a pressurecompensation degree of the compressors, and for calculating thecompressors' switch-on response times at stopped status or in theno-load state.

Moreover, a further measuring means can be provided supportingly whichis likewise connected to the central control means and ascertains themeasured pressurized fluid consumption, respectively the withdrawal ofpressurized fluid in order to determine e.g. switch-on response timeswith higher accuracy.

The compressors' operating data exchanged with the central control meansvia the communications bus 5 inter alia concern the current operatingstate of each compressor. This information is required by thecontrolling or regulating method among other things for selecting thecompressors to be switched to load. Furthermore, this informationcomprise the motor speed based on which the control method can ascertainthe energy consumption of a compressor or a compressor group. Same canfurther include information on compressor-internal pressure sensors forassessing after-running times, when the compressor e.g. is at no-loadrunning, respectively expected after-running times, when the compressoris at load running, as well as information on whether the compressor isin load operation at all or not. Alternatively, some or all of thementioned operating data of the compressors can also be remodeled orapproximated by way of data processing so that same do not need to beexchanged via the communications bus 5 and are all the same available tothe central control means in sufficient approximation.

For application-technological reasons, the compressor system 1 canmoreover comprise editing elements 21 which give cause to acharacteristic change of the system-internal fluid pressures. Theinfluence of the editing elements 21 within the compressor system 1,however, can be appropriately compensated for by a suitable adaptivelearning behavior of the control or regulation. An increasing time delayin the pressurized fluid conveying between a compressor and the centralpressurized fluid reservoir due to a filter being increasinglycontaminated, in the form of an increasing switch-on response time ofthe compressor from the off-state as well as the no-load running statecan, for instance, be adaptively compensated. Such an increasingswitch-on response time can be simply compensated by the control so thatthe increasing filter contamination does not affect the maintaining ofthe predefined overpressure within the pressurized fluid reservoir 3.

Based on application-technological considerations, the compressor system1 can moreover comprise one or more pressure regulating valves forpressure stabilization.

FIG. 2 shows a schematic representation of the control method of controlmeans 4. The control means 4 is in this case in communicative contactwith the communications bus 5 and can both read in and out data. Thecontrol means 4 can optionally transmit switching commands to singlecompressors 2 via the communications bus 5. For the feeding of controlparameters, respectively inputting of data for characterizing thecompressors 2, the control means 4 includes a feeding interface 40. Saiddata is transmitted to the control method 41 which can be implemented asa software application in terms of an adaptive control method. Thecontrol method 41 generates suitable control commands, respectivelyswitching commands for controlling the compressors 2 which aretransmitted to the compressors 2 via the communications bus 5. Thecontrol method 41 includes a control algorithm 42 for this purpose whichoptimizes the energy demand of the compressor system within a pressuretolerance range between adaptation pressure 101 and upper pressure limit104. It is to be noted here that the control algorithm 42 can also beunderstood as a regulating algorithm. The control means 4 comprisesfurther a presently not shown system clock having an appropriate timerwhich is capable of provide an appropriate timing to the control method41.

In accordance with the embodiment, the control algorithm 42 permits anenergy-guided adaptive regulating and assesses a switch-off pressure 103for the compressors to be switched off load within the availablepressure tolerance range in an energetically targeted manner. For thispurpose, the control algorithm 42 calculates the energetically optimumswitch-off pressure 103 in a mathematically analytical form. Thisoptimum switch-off pressure 103 is defined in accordance with theembodiment by the minimum value of a function which describes the totalpower loss of all of the compressors 2 of the compressor system 1 duringone switching cycle depending on the switch-off pressure 103. Inaccordance with the embodiment, then assumption enters here into thecalculation that the withdrawal of pressurized fluid remains on averageconstant and that the switching cycle therefore repeats uniformlybetween two successive minimum, respectively maximum pressure values.The assumption of a pressure fluid withdrawal being on average constantallows for pressure profile fluctuations to be taken into account in thereal pressure profile as well.

In doing so, the energy-guided control algorithm 42 makes use of presentregulation-technological degrees of freedom in that these degrees offreedom are not occupied or limited by fixedly predetermined controlparameters or else a too small or strictly predetermined pressureregulating range, but optimizes same with respect to energy. Both theselecting of the compressors 2 to be switched and the points in time,respectively pressures for the switching operations to be implementedare not parameterized but are calculated by the control method 41 on acase-by-case basis in en energetically optimized manner.

Apart from the energy-guiding of the control method 41, same is alsocharacterized by an adaptive behavior with respect to adapting adaptiveparameters during running operation. In doing so, the adaptive behaviorsupports the optimizing of the energy demand of compressor system 1decisively. The adaptive behavior is based on an adaptation algorithm 43comprised by the control method 41 which adjusts all adaptive parametersduring the compressor system's operation and makes them available to thecontrol algorithm 42. The adaptive behavior also allows the selecting ofthe compressors to be switched to being automatically adapted toregulation-relevant, fixed and variable characteristics respectivelyconditions of the compressor system as well as the use thereof in therunning operation. Examples of such adaptive parameters can be theenergy demand per conveyed fluid amount of a compressor 2, as well asthe pressure-technologically active storage volume of the pressurizedfluid system and the temporal switching behavior of the compressors 2.

FIG. 3 shows a schematic representation of the sequence of single stepsin accordance with one embodiment of the inventive method forcontrolling a compressor system 1 in a flowchart representation. In thiscase, determined switching alternatives 13 are excluded in apre-selecting step 10 in an excluding means 6 of a control means 4 notshown in greater detail from the multitude of combinatorially availableswitching alternatives 13, preferably while taking into account thecurrent conditions. The pre-selecting e.g. can be performed on the basisof selection criteria taking into account the technical feasibility ofthe predetermined switching alternative 13. In the present case, forinstance, eight combinatorially possible switching alternatives 13 areavailable in total, from which four switching alternatives 13(deselected by crossing out) have shown to be inappropriate for thepresent operating conditions and hence are deselected in advance. Fromthe remaining four switching alternatives 13, one switching alternative13 is selected in a main selecting step 11 in a selecting means 7 of thecontrol means 4 not shown in greater detail while referring to one ormore optimizing criteria by weighing up all of the switchingalternatives 13 against one another which had not been deselected in thepre-selecting step 10. The selected switching alternative 13 assessed inthe main selecting step 11 is output in a control step in an outputmeans 8 of the control means 4 not shown in greater detail for beingimplemented in the compressor system 1. In the present case, theoutputting has been illustrated symbolically as a forwarding ofinformation from the output means 8 to the communications bus 5 which,however, should not be understood here as being limiting.

FIG. 4 shows a representation of the real pressure profile 105 in thepressurized fluid system during a periodic time interval T_(switch). Thelength of the periodic time interval T_(switch) here is related to justthe length of one switching cycle. In accordance with one embodiment ofthe inventive control method, the control assesses an individualswitch-off pressure 103 for the compressor 2 to be switched from loadwithin the available pressure tolerance range according to principles ofenergetic optimization. The pressure tolerance range is in this case thepressure range between an adaptation pressure 103 not to be undercut andan upper pressure limit 104 not to be exceeded. In accordance with theembodiment, the energetically optimum switching cycle pressuredifference and the energetically optimum switch-off pressure 103 for thecompressors 2 to be switched to load is mathematically-analyticallycalculated as an energetic optimum. For this calculation will be assumedthat the withdrawal of pressurized fluid is on average constant. Thepressure drop can consequently be represented as a rise of a linearlyfalling straight line which approximately describes the real pressureprofile. Analogously, the increase of pressurized fluid within thepressurized fluid system can be described by a largely similarmathematical averaging of the really rising pressure profile as amonotonously rising straight line.

Under these presuppositions of on average constant withdrawal ofpressurized fluid, the switching cycle including the next switch-onoperation can be energetically described by a simple mathematicalrepresentation. Due to this simple mathematical representation it ispossible to calculate the energetic optimum respectively the maximumefficiency of the compressor system during such a switching cycle. Forthis purpose, the control method 41 regulates the switch-off pressure103 of the compressors 2 to be switched such that the entire total powerloss depending on the switching cycle (total work loss per periodic timeinterval T_(switch)) becomes minimal.

Both the load-running as well as the switching and idle runningcompressors 2 contribute to this power loss depending on the switchingcycle. The energy demand of the load-running compressors (load work)increases with the switching cycle pressure difference since theinternal working pressure difference thereof increases on average. Incontrast hereto, the switching work loss as well as the no-load workloss of the compressors to be switched decreases with an increasingswitching cycle pressure difference since the number (frequency 9 ofswitching cycles decreases. The sum of loss components in an energeticoptimization occupies a minimum in the calculated switching cyclepressure difference. The expression to be minimized result in accordancewith the following equation (1):

P _(V)=(ΔW _(load) +ΔW _(no-load) +ΔW _(switch))T _(switch)  (1)

In this case, ΔW_(load) is the work loss of the load-running compressorsper switching cycle due to the pressure elevation as compared to theswitch-on pressure, ΔW_(no-load) is the no-load work loss of thecompressors to be switcher per switching cycle due to the no-loadperformances and after-running time thereof, ΔW_(switch) is theswitching work loss per switching cycle of the compressors 2 to beswitched due to the slow internal pressure compensation process duringswitching in no-load running, probably of a motor restart, and theinternal pressure adaptation when switching to load, T_(switch) is theduration of a switching cycle which temporally extends over a periodicpressure increase and the subsequent pressure drop.

The individual components of the total work loss are in this casecalculated in accordance with equation (2):

ΔW _(load)=0.5•r _(load) •Δp _(switch) ²·•(P _(load1) /ldp/dtl_(average1) +P _(load2) /ldp/dtl _(average2))  (2)

In this case, r_(load) is the relative increase of the load performanceof load-running compressor 2 per pressure unit, Δp_(switch) is theswitching cycle difference, P_(load1) is the load performance of thecompressors, the compressors 2 to be switched included, which areload-running toward the switch-off pressure 103 in the course of thepressure profile at the switch-on pressure 102, ldp/dtl_(average1) isthe amount of the expected average pressure increase during the realpressure profile toward the switch-off pressure 103, calculated on thebasis of a commensurate period of time, P_(load2) is the loadperformance of the compressors, the compressors 2 to be switchedexcluded, which are load-running toward the switch-off pressure 103 inthe course of the pressure profile at the switch-on pressure 102,ldp/dtl_(average2) is the amount of the expected average pressureincrease during the pressure profile toward the switch-on pressure 102from ldp/dtl_(average1) and the pressure compensating effect of thecompressors 2 to be switched.

The no-load work loss ΔW_(no-load) is calculated on the basis of thefollowing equation (3):

ΔW _(no-load)=Σ(P _(no-load) •T _(no-load))  (3)

In this case, P_(no-load) is the no-load performance of the individualcompressors to be switched, and T_(no-load), the after-running time atno-load of the individual compressors to be switched, is restricted to atime between the switching on and off.

The switching work loss ΔW_(switch) is calculated as a sum of theswitching work losses W_(switch) per switching cycle of the compressors2 to be switched in accordance with the following equation (4):

ΔW _(switch) =ΣW _(switch)  (4)

Furthermore, the periodic time interval T_(switch) of a switching cyclecan be easily calculated in accordance with equation (5) based on thefollowing correlation which results from simple geometric considerationsas per FIG. 4:

T _(switch) =ΔP _(switch)·•(1/dtl _(average1)+1/ldp/dtl_(average2))  (5)

The calculation of the energetically optimum switching cycle pressuredifference Δp_(switch,opt) can be calculated using equation 81) bysimply inserting the terms for the individual work losses ΔW_(load),ΔW_(no-load), ΔW_(switch) as well as the length of the periodic timeinterval (switching cycle duration) T_(switch) into the formula as perequation 1 for the power loss P_(V) which depends on the switchingcycle, by subsequently deriving according to the switching cyclepressure difference Δp_(switch) and correspondingly zero-setting of thederivation. Consequently, the energetically optimum switching cyclepressure difference Δp_(switch,opt) can be represented as amathematically easy to handle expression in accordance with equation(6):

Δp _(switch,opt)=√{[Σ(P _(no-load) •T _(no-load))+ΣW _(switch)]/[0.5•r_(load)•(P _(load1) /ldp/dtl _(average1) +P _(load2) /ldp/dtl_(average2))]}  (6)

The energetically optimum switch-off pressure in applications ofpressurized fluid results as a sum of the adaptation pressure 101 andthe calculated energetically optimum switching cycle pressure differenceΔp_(switch,opt). In corresponding vacuum applications, for example, theenergetically optimum switch-off pressure 103 results as the differenceof the two previously mentioned values as will be clear to the personskilled in the art.

It should moreover be pointed out that the control method in accordancewith the embodiment takes into account the delay times of the individualcompressors 2 or combinations of compressors 2 which are determined fromthe times between the switching on or off of a compressor 2 and thepoints of time of the actual implementation of the change of state.Accordingly, the switch-on times T_(on) just as the switch-off timesT_(off) are temporally advanced in comparison to the minimum pressurevalues of the real pressure profile 105 respectively the maximumpressure values.

Furthermore, FIG. 4 shows a partly idealized switching cycle forillustrative purposes. An upper pressure limit 104 is defined bysystem-contingency, e.g. by the components' pressure resistance. Thelowermost line in the diagram represents the adaptation pressure 101which already had been discussed several times. The pressure profile inthe switching cycle illustrated here moves between a (local) minimumvalue P_(min) and a (local) maximum value P_(max). At a point of timeT_(AB), namely upon reaching the switch-off pressure 103 at a risingpressure profile, measures are taken for reducing the generating ofcompressed pressurized fluid, which have the effect that the pressureshortly rises above the switch-off pressure 103 to the (local) maximumvalue P_(max) but then the pressure increase reverses into a pressuredrop. Once the switch-on pressure 102 is reached at a falling pressureprofile, measures are taken for increasing the generating of compressedpressurized fluid so that the pressure further decreases to a (local)minimum value P_(min) but the pressure drop then reverses into a newpressure increase.

It is to be noted at this point that all of the above describedcomponents, whether alone or in any combination, are claimed as beingessential to the invention, optionally the details depicted in thedrawings. Variations thereof will be familiar to those skilled in theart.

LIST OF REFERENCE NUMERALS

-   -   1 compressor system    -   2 compressor    -   3 pressurized fluid reservoir    -   4 control means    -   5 communications bus    -   6 excluding means    -   7 selecting means    -   8 output means    -   9 switch-off pressure determining means    -   10 pre-selecting step    -   11 main selecting step    -   12 control step    -   13 switching alternative    -   20 measuring means    -   21 processing element    -   22 functional element    -   30 data record    -   40 feeding interface    -   41 control method    -   42 control algorithm    -   43 adaptation algorithm    -   101 adaptation pressure    -   102 switch-on pressure    -   103 switch-off pressure    -   104 upper pressure limit    -   105 real pressure profile    -   T_(on) switch-on time    -   T_(off) switch-off time

1-28. (canceled)
 29. A method for controlling a compressor systemcomprising a plurality of compressors, each compressor optionally havinga different design or performance, the compressor system maintaining apredefined excess pressure in a pressurized fluid system despite awithdrawal of pressurized fluid from the pressurized fluid system, themethod comprising: increasing generation of compressed pressurized fluidupon reaching an optionally variable switch-on pressure, wherein theswitch-on pressure is higher than an adaptation pressure which is not tobe undercut; and reducing generation of compressed pressurized fluidupon reaching a variable switch-off pressure, wherein the switch-offpressure is determined by periodically and computationally minimizing aquotient of a total work loss in a predefined periodic time intervalconcerning one switching alternative, and the periodic time intervalitself, the periodic time interval being based on virtual switchingcycles, in particular representing a time for a pressure profile to risefrom a minimum pressure value to a maximum pressure value andsubsequently decrease to the minimum pressure value.
 30. The methodaccording to claim 29, wherein the switch-off pressure is assessed orcalculated on a case-by-case energy optimization basis.
 31. The methodaccording to claim 29, wherein an optimum switch-off pressure iscalculated based on the following formula:Δp _(switch,opt)=√{[Σ(P _(no-load) •T _(no-load))+ΣW _(switch)]/[0.5•r_(load)•(P _(load1) /ldp/dtl _(average1) +P _(load2) /ldp/dtl_(average2))]} with W_(switch) as switching work loss, P_(no-load) asno-load performance of individual compressors to be switched,T_(no-load) as after-running time at no-load of individual compressorsto be switched, r_(load) as relative increase of load performance ofload-running compressor per pressure unit, P_(load1) as load performanceof the compressors to be switched included, ldp/dtl_(average1) as amountof the expected average pressure increase during the real pressureprofile toward the switch-off pressure, P_(load2) as load performance ofthe compressors to be switched excluded, and ldp/dtl_(average2) asamount of the expected average pressure increase during the pressureprofile toward the switch-on pressure from ldp/dtl_(average1) and thepressure compensating effect of the compressors to be switched.
 32. Themethod according to claim 29, wherein the switch-off pressure iscalculated based on an energy demand of one or more of load runningcompressors optionally when supplying against a continuously increasingpressure; no-load running losses of the load running compressorsswitched to no-load running or stopped status; no-load running losses ofno-load running compressors; or switching loss energy of the compressorsswitched based on the selected switching alternative.
 33. The methodaccording to claim 29, wherein the switch-on pressure is calculated suchthat an actual pressure profile reaches a calculated adaptationpressure, which is below the switch-on pressure at a deviation of lessthan 5%.
 34. The method according to claim 29, wherein determining ofthe switch-off pressure or switch-on pressure is performed in real-time.35. The method according to claim 29, wherein control of the compressorsystem is based on empirical parameters from past switching operations.36. The method according to claim 35, wherein the empirical parametersinclude one or more of: a level of energy demand of each of thecompressors, switch-on response times of the compressors, consumptionbehavior of consumers of the pressurized fluid, a size of a pressureaccumulator, or a pressure compensation degree of one or morecompressors.
 37. The method according to claim 35, wherein the empiricalparameters include one or more of: a pressure compensation degree of acompressor depending on the storage volume and an installation scheme ofthe pressurized fluid system; a level of energy demand of a compressordepending on a previous mode of operation of the compressor,environmental temperature, maintenance, wear and contaminating states;or a switch-on response time and pressure compensation degree of acompressor depending on typical patterns of change of the withdrawal ofpressurized fluid.
 38. The method according to claim 29, wherein underpredefined conditions, switch-on or switch-off commands triggered upon areal pressure profile reaching the switch-on pressure or the switch-offpressure are suppressed or additional switch-on commands or switch-offcommands are triggered independent of the real pressure profile reachingthe switch-on pressure or the switch-off pressure.
 39. A non-transitorycomputer readable storage medium having stored thereon computerexecutable instructions which, when executed on a computer, configurethe computer to perform a method for controlling a compressor systemaccording to claim
 29. 40. The method according to claim 29, wherein theswitch-on pressure is calculated such that an actual pressure profilereaches a calculated adaptation pressure, which is below the switch-onpressure at a deviation of less than 2%.
 41. The method of claim 29,wherein the switch-off pressure is a sum of the adaptation pressure andthe calculated energetically optimum switching cycle pressure differenceΔp_(switch,opt).
 42. The method according to claim 29, whereindetermining a current value of the withdrawal of pressurized fluid iseither determined by a measuring device or calculated from a past realpressure profile, an operating state of the compressors, or anadaptively adjusted accumulator size of the compressor system.
 43. Themethod according to claim 29, wherein a determined switching off is onlytriggered by the control when a necessary switch-on operation can beperformed in time in consideration of a startup behavior of a possibleswitching combination.
 44. The method according to claim 29, wherein aswitching on of one or more compressors is performed such that inconsideration of the startup behavior of the one or more compressors andin consideration of the preferably adaptively learnt switch-on responsetimes of each of the compressors, a real pressure profile reaches theadaptation pressure at a deviation of less than 2%.